Originally posted by Pete VanderLaan When you say it is dead on, who measured it and how. Is it a pull test? I simply do not trust pull tests in opal/clear matches as even remotely reliable. They only work in formulas of similar composition. It would require dilatometry or a ring or trident seal test to get accuracy here.

Just so we're all straight on what's being measured:

a DILATOMETER is used to measure the expansion, e.g. COE(0-300C) or “LEC”. This is the measurement of an isolated glass.

The following three tests measure the combined effect of expansion AND viscosity on two different glasses:

RING TEST

TRIDENT SEAL TEST

BULLSEYE’S CHIP/BAR TEST

The PULL TEST is also a rough measure of the combined effect of expansion and viscosity on two different glasses, BUT in our factory experience it is very frequently unreliable due to on the skill level (or lack thereof) of the person performing the test.